Glow cathode and electron tube containing same



Patented 24, 31928.

UNITED STATES A ENT "c rrus;-

ERN ST 'IBIEDEBICH, OI IBERLIN-CEARLOTTENBUBG, GERMANY, ASSIGN'OB T BADId COBROBATION OI AMERICA, A CORPORATION OF DELAWARE.

Grow cannons AND 'nmiornon TUBE con'ramme Io Drawing. Application filed November 28, 1925, Serial No. 72,044, and in- Gernumy December 1, 1981.

My invention refers to electron tubes and more especially-to the glow cathodes form'- ing part of such tubes. It isan object of my invention to provide a'cathode and .a

a tube embodying suchcathode which is more eflicientthan slmilar articles hitherto used.

In the'glow cathode forming art of electron tubes tungsten wire and wires of other metals, have been used which contain addil0 tions of.oxides-having a high meltin point.

By .such addition the emission of eectrons' is increased far beyond the emission takingplace in the case-of pure metal wire. It is also known to still further increase the ciliciency of glow cathodes containing such additions bysubjectin "them to a treatment of reduction with meta s of an alkali or an alkaline earth or with carbon. It is this latter method which has been-practiced on a large 2 scale; however, the treatment with carbon. entails the drawback that the tungsten wirecombines with the carbonto-form'a carbide, wherebyit loses its elasticity and gets brittlef The chances of these wires breaking during transport or during thense of these tubes are very great.

It has now been ascertained that if the pure tungsten wire is enveloped in a skin or coating of thorium carbide, an extraordif narily favorable emission of electrons is obtained, which is even superior to the emission obtained with tungsten wire containing thorium oxide which has been subjected to reducing treatment. "This fact could not be :5 foreseen as it could not be assumed that thorium carbide would have the same electron emission as metallic thorium. It has now been ascertained by extensive investigation that this is the fact. p

- y The glow cathode according to this inventionis ametal wire, for instance a tungsten wire, capable of high electron emission and which is provided with a skin or coating containing a carbide. My invention further relates to the method of producing such cath odes and the tubes of which they formpart. In the production of cathodes according to this invention I have found it particularly useful to employ a mixture of ThO' -litl,

using as a binder a solution of acetylic cellulose in an organic solvent. It is an easy matter to coat a wire several hundred meters long with a coating consisting of such mixture, the coating procedure being well 58 known so as not to require further escrip maining on the surface of the wire when the tube is supplied with current, which can be done either when it is still connected with the vacuum pump. or also after the melting ofi'. Tubes manufactured in this way fur- The manufacture ofthe tubes by nishalready at low,,jtemperature an emission equal to that of a wire having the same surface, but-*lackin the coatin but such tubes enjoy a very long life an all danger ofnbreaking of the wire is done away with inasmuch as the tungsten wire itself carrying theskin or coating does, not contain any carbon.

As a rule tungsten wire coated with" I thorium carbide has proved to be useful. I have, however, ascertained that also other carbides can. be used with advantage, for in-- stance the carbides of the metals of the second, third and fourth groups of the periodical system of elements, which are known'to have a high electron emitting capacity, and i also .the carbides of the rare earths. In these cases it can be proved advantageous to replace thetungsten by-another metal acting as carrier, for instance a noble metal such as platinum.

As a rule pure metals will be used as carriers for the carbides. It may, however, happen sometimes that the carbide coating is prematurely removed in some place either from mechanlcal causes or by evaporation in consequence of overloading, so that in consequence of a different emission of electrons or different radiation there arise difierences of temperature whereby single places are overloaded and cause the cathode to burn through prematurely. In order to avoid this as far as possible I have found it useful to employ a carrier wire which already contains a small quantity of carbon, as a rule not exceeding 0.1 percent. I hereby revent the difference in the emission of e ectrons from becoming too eat and I prevent the wire from burning t rough.

desire to be limited to the exact substances, proportions, temperatures and operations described, for obvious modificationswill occur to a person skilled in the art.

I claim v 1. A glow cathode for electron tubes comprising a metal Wire containing a small percentage of carbon and a coating on this wire containing a carbide of a metal capable of high electron emission.

2. A glow cathode for electron tubes com prising a wire of tungsten containing a small percentage of carbon and a coating on this wire containing thorium carbide.

3. A cathode for electron tubes comprising a Wire of tungsten containing a small quantity of carbon not exceeding 0.1 percent and a skin 01' coating on this wire containing thorium carbide.

4. The method of forming a cathode which tron emitting capacity and heating the resulting product sufficient to form a carbide of the metal.

5. The method of forming a cathode which consists in coating a refractory core with a mixture of thorium oxide and carbon and heating the resultant product until a coating of thorium carbide is formed on the core.

6. The -.method of coating a filamentary cathode which consists in applying to the surface thereof a mixture of thorium oxide, carbon and a binder, and heating the cathode to decompose the binder, reduce the thorium oxide and form a film of thorium carbide on the surface of the cathode.

In testimony whereof I affix my signature.

ERNST FRIEDERICH. 

